Australian scientists have discovered the world's oldest oil in rocks that date back 3.2 billion years.

The find suggests that oil-forming micro-organisms were widespread very early in the Earth's history.

The researchers say that oil-forming bacteria may have been among the earliest inhabitants of our planet and that the sulphur-springs that formed the rocks studied by the researchers may have been the "cradle of life on Earth".

The scientists are now looking for molecular fossils that might be present in the oil.

Dr Birger Rasmussen, of the University of Western Australia, and Dr Roger Buick, of the University of Sydney, have published details of their work in the journal Geology.

The tiny droplets of oil they found were extracted from rocks formed in an ancient sulphur spring that left behind huge deposits at a site in Australia.

The minute droplets of oil are at least 250 million years older than similar droplets found by the same team in 1998. Before these discoveries, the oldest known oil had been dated as 1.5 billion years old.

Fluid inclusions

The researchers found oil preserved within fluid inclusions, microscopic-sized droplets of fluid trapped within mineral grains, similar to gas bubbles trapped in ice cubes.

The inclusions measure less than a hundredth of a millimetre across and are detectable by their fluorescence when exposed to ultraviolet light.

The hydrocarbons in the ancient oil must be a decay product of the living creatures that were around on earth 3.2 billion years ago.

The oil leads scientists to speculate that oil generation early in Earth's history was widespread and that aquatic life at the dawn of evolution was more abundant than previously thought.

A detailed analysis of the droplets could yield valuable information about the early biosphere, especially if they turn out to contain molecular fossils of the primordial organisms from which the oil was made.

Sulphur springs, like the one that laid down the sulphur-rich rocks looked at in this study, may be "the cradle of life itself", according to the scientists.

This research is also important for oil companies as it suggests that exceptionally old rocks may contain oil reserves. To date, such rocks have not been a priority for oil prospectors.